Magnetic resonance imaging system with a plurality of transmit coils

ABSTRACT

The invention relates to an MRI system ( 1 ) for nuclear magnetic resonance imaging which comprises a plurality of transmit coils ( 11, 12 ). Each coil receives a coil drive signal (SD 1 , SD 2 ). The respective coil drive signals have the same shape, but may have a different amplitude and phase, controlled by a controller ( 103 ) on the basis of characteristic information in a memory ( 104 ) as well as user input information. The controller is designed to set the respective amplitudes and phases in such a way that the resultant overall B1 field is as homogeneous as possible in a volume of interest.

The invention relates to a magnetic resonance imaging (MRI) systemcomprising:

an object space for receiving an object to be examined;

a main magnet system for generating a main magnetic field in the objectspace;

a gradient magnet system for generating gradients of the main magneticfield in the object space;

a plurality of transmit coils located adjacent the object space;

a coil drive circuit for generating a plurality of individual coil drivesignals.

In the magnetic resonance imaging (MRI) technique, proton spins of abody under examination, for example a human body, are excited; afterexcitation, the spins return to their equilibrium state and in thisprocess they transmit an electromagnetic field which is called a freeinduction decay signal (FID). This FID signal can be received and MRimages derived therefrom. Since the MR imaging technique is well knownper se and the present invention does not relate to the MR imagingtechnique as such, the MR imaging technique will not be explained inmore detail, herein.

In the magnetic resonance imaging technique, a magnetic field is appliedto the object under observation, the magnetic field having severalcomponents. The B0 field is a strong, static field which aligns thespins in a state of equilibrium. The B0 field is a high frequency field(normally a pulsed field) which excites the spins out of their state ofequilibrium. The frequency of the B1 field depends on the application;it is usually in the radio frequency range (RF). Furthermore, gradientfields Gx, Gy and Gz are applied.

The B1 field may have components in X- and Y-directions, perpendicularto each other and to the B0 field direction. The B1X and B1Y fields mayexhibit a certain predetermined phase relationship with respect to eachother.

As is commonly recognized, it is desirable that the B1 field ishomogeneous or uniform within a certain measuring volume. This meansthat the spins of the nuclei in a volume of interest are all excited tothe same extent by the magnetic field.

MRI systems comprise transmit means, including transmit antennas orcoils, for generating the magnetic field to be applied to the body underexamination, and receive means, including receive antennas or coils, forreceiving the signals transmitted by the nuclei of such a body. Thedesirability of a homogeneous B1 field implies the desirability of atransmit antenna having a homogeneous transmit characteristic.Furthermore, for measuring integrity it is desirable that the receiveantenna has a homogeneous sensitivity characteristic, meaning that thereceive antenna is sensitive to the same extent to all nuclei within thevolume of interest. If the receiver has an inhomogeneous sensitivitycharacteristic, it is usually possible to compensate for this aspect insubsequent image processing. However, if the transmit antenna has aninhomogeneous sensitivity characteristic, one consequence will be thatdifferent portions within the volume of interest will be excited in adifferent manner; the differences in excitation may then depend on thedeviations from homogeneity in a nonlinear way. This may lead to a lossof contrast in some portions of the volume of interest.

Therefore, a general objective of the present invention is to provide anMRI system of the kind mentioned in the opening paragraph with improvedhomogeneity of the B1 field.

A complicating factor in this respect is that the object in the volumeof interest may have an effect on the B1 field. Due to its electricalproperties, this is especially the case for human tissue. Even if thetransmit antenna were to have an ideal homogeneous characteristic, themagnetic field within the object under observation might beinhomogeneous due to distortions caused by the object itself. Suchdistortions may be due to, for example, internal resonances within theobject, or to absorption by the object.

A usual approach for compensating absorption is to increase transmitpower. However, one obvious disadvantage is an increase in powerdissipation in the object under investigation, which is especiallyundesirable in the case of examination of a human patient.

Therefore, the present invention aims to improve the homogeneity of theB1 field without substantially increasing overall transmit power,preferably even while reducing overall transmit power.

The desirability of a uniform B1 field has already been recognized inprior art. Previously proposed solutions include, for instance, the useof composite RF pulses or the use of adiabatic pulses. Both approachesinvolve a substantial increase in RF dissipation within the object underobservation. Furthermore, composite RF pulses can only be used for alimited number of pulse types, for instance 90° pulses and 180° pulses;composite RF pulses do not solve the problem of providing, for instance,homogeneous 30° pulses.

U.S. Pat. No. 6,049,206 describes a complicated method which involvesproviding an initial, non-homogeneous B1 pulse and an additional pulsewhich consists of a phase modulation of the initial B1 pulse and has atime-dependent phase relationship with respect to the initial B1 pulse.Such an approach, besides being complicated, is only suitable forspecific pulse types, specifically 90° pulses and 180° pulses.

An object of the present invention is to provide a magnetic resonanceimaging system of the kind mentioned in the opening paragraph in whichthe homogeneity of the B1 field is improved with relatively simplemeans.

In order to achieve said object, a magnetic resonance imaging (MRI)system in accordance with the present invention is characterized in thatthe individual coil drive signals are generated by the coil drivecircuit so as to have a substantially identical shape, the system havingcontrollable means for individually setting the amplitude and/or phaseof each of said coil drive signals, and a controller for controllingsaid controllable means. In an MRI system according to the invention thetransmit means comprise at least two transmit antennas or coils. Theindividual antennas are driven by an RF pulse derived from one basicsignal, but weighted by individual weighing factors, in such a way thatthe resultant overall B1 field is substantially homogeneous within thevolume of interest.

These and other aspects, features and advantages of the presentinvention will be further explained by the following description of thepresent invention with reference to the drawings, in which correspondingreference numerals indicate corresponding or similar parts, and inwhich:

FIG. 1 schematically illustrates an arrangement of two coils and theresultant magnetic field in an object space;

FIG. 2 is comparable to FIG. 1, illustrating the effect of the inventionon the homogeneity of the magnetic field; and

FIG. 3 is a block diagram schematically illustrating an embodiment of acoil drive circuit.

FIG. 1 schematically illustrates an MRI system 1 according to theinvention which is used to form images of the intestines of, forexample, a human body by means of nuclear magnetic resonance (NMR)techniques. The MRI system 1 has an object space 2 for receiving anobject 3 to be examined. The MRI system 1 also comprises a main magnetsystem for generating a main magnetic field in the object space 2, and agradient magnet system for generating gradients of the main magneticfield in the object space 2. The main magnet system and the gradientmagnet system are not shown in FIG. 1 because the exact structure anddetails of the main magnet system and the gradient magnet system are notrelevant for the present invention. The main magnet system and thegradient magnet system may be of a kind known to and generally used by aperson skilled in the art of magnetic resonance imaging systems. The MRIsystem 1 comprises first and second transmit antennas 11 and 12,hereinafter indicated briefly as “coils”, each designed for generatingan RF magnetic field. The two coils 11 and 12 are located on oppositesides of the object space 2. An object located in the object space 2 isgenerally indicated by the reference 3; this object may for instance bea human body. An object part within the object 3 is generally indicatedby the reference 4; this object part may for instance be a human liver.In the following explanation it is assumed that it is desired to obtainan image of the liver of a human being; thus, a volume of interest 5 isdefined by object part 4. The volume of interest 5 may in principle beidentical to the volume occupied by the liver, but in this case, foreasy reference, the volume of interest 5 is taken to be slightly largerthan the volume of the liver 4.

FIG. 1 also shows a graph containing curves 21 and 22 indicating thelocal field strength of the magnetic field generated by the first coil11 and the second coil 12, respectively. The horizontal axis of thisgraph indicates location and is aligned with the schematic drawing ofthe MRI system 1.

It can clearly be seen from the curve 21 of this graph that the firstcoil 11 generates a non-homogeneous field having its highest intensitycoinciding with the location of the first coil 11 and generallydecreasing with distance. Especially the magnetic field generated by thefirst coil 11 is not homogeneous at the location of the volume ofinterest 5 (see part 21 a of the curve 21).

Likewise, it can clearly be seen from the curve 22 of this graph thatthe second coil 12 generates a non-homogeneous field having its highestintensity coinciding with the location of the second coil 12 andgenerally decreasing with distance. Especially the magnetic fieldgenerated by the second coil 12 is not homogeneous at the location ofthe volume of interest 5 (see part 22 a of the curve 22).

It is noted that, in this example, the curves 21 and 22 are identical;however, although such is preferred, it is not essential for the presentinvention.

The overall B1 field generated by the coils 11 and 12, i.e. a directsummation of the fields 21 and 22, is shown at 20 in the graph ofFIG. 1. In the state of the art, both coils 11 and 12 generate the samefield strength, i.e. they receive substantially the same amount ofpower, as illustrated by the curve 20. It can clearly be seen in thiscase that the B1 field 20 is not homogeneous at the location of thevolume of interest 5 (see part 20 a of the curve 20). The B1 field 20has a minimum, around which the B1 field is substantially homogeneous,but this minimum has a fixed location within the object space 2, whichlocation does not necessarily correspond to the location of the volumeof interest 5.

FIG. 2 is comparable to FIG. 1, except that the graph illustrates asituation where the overall power applied to the coils is redistributedsuch that the first coil 11 receives more power and the second coil 12receives less power as indicated by the first field curve 21 which israised and the second field curve 22 which is lowered relative to theirpositions in FIG. 1. The redistribution of power can be done such thatthe overall amount of power remains the same. According to theprinciples of the present invention, the redistribution of power is donein such a way that the B1 field 20 is as homogeneous as possible at thelocation of the volume of interest 5 (see part 20 b of the curve 20).

It is noted that in the example of the FIGS. 1 and 2 two coils 11 and 12are used for illustrating the principles of the present invention.However, the present invention is not restricted to the use of twocoils; in fact, the number of coils may be any number larger than one,although in practice the number will not be very large.

It is further noted that in the above example only the effect ofrelative amplification/attenuation of the two coil signals is discussed.In practice it may also be appropriate to introduce a relative phaseshift between the two coil signals in order to compensate for relativedelays caused by differences in propagation velocity of the field in theobject under observation.

FIG. 3 schematically illustrates an embodiment of a coil drive circuit100 for implementing the above coil drive method in the MRI system 1. Asignal generator 101 generates a basic signal SB. If required, anamplifier 102 amplifies this basic signal SB; such an amplifier may alsobe incorporated in the signal generator 101. Since such a signalgenerator for generating a basic nuclear magnetic resonance (NMR) drivesignal is commonly known and the present invention can be implementedusing a prior art signal generator, it is not necessary here to discussthe design of such a generator in more detail. Moreover, since asuitable shape of a basic NMR drive signal is known to persons skilledin this art, it is not necessary either to discuss such a shape in moredetail.

The coil drive circuit 100 comprises a plurality of coil drive branches110, 120, etc for driving the plurality of coils 11, 12, etc. In thisexample only two coils 11 and 12 are discussed; therefore, only twocorresponding branches 110, 120 are shown. Each coil drive branch 110,120 comprises a series arrangement of a controllableamplifier/attenuator 111, 121 and a controllable phase shifter 112, 122,controlled by a controller 103 which has an associated memory 104. Inthe example shown, the phase shifter is always arranged behind theamplifier, but this order may also be reversed.

Each branch 110, 120 has its input side (in this case the input ofamplifier/attenuator 111, 121) coupled to the output of the generatoramplifier 102. Each amplifier/attenuator 111, 121 amplifies orattenuates its input signal with a gain factor G1, G2 under the controlof the controller 103 so as to provide an amplified signal S_(BA1)=G1·SBand S_(BA2)=G2·S_(B), respectively. Each phase shifter 112, 122generates an output signal S_(D1) and S_(D2), respectively, which issubstantially identical to its input signal S_(BA1) and S_(BA2),respectively, but delayed by a delay δ1, δ2 under the control of thecontroller 103. The output signals SD1 and SD2, respectively, areapplied to the coils 11 and 12, respectively. Thus, the coils 11, 12 aredriven by coil drive signals SD1, SD2, respectively, which can bewritten as:S _(D1)(t+δ1)=G1·S _(B)(t)S _(D2)(t+δ ₂)=G ^(2·) S _(B)(t)wherein t represents time.

The controller 103 is designed to control the gain G1 and G2 applied byamplifier/attenuator 111 and 121, respectively, and the phase shifts δ1and δ2 applied by the phase shifter 112 and 122, respectively, in such away that the coils 11, 12 receiving said output signalsS_(D1)(t+δ1)=G1·S_(B)(t) and S_(D2)(t+δ2)=G2·S_(B)(t), respectively,generate magnetic field contributions 21, 22 such that the resultantoverall magnetic field 20 is as homogeneous as possible in the volume ofinterest 5 (see 22 b in FIG. 2). In order to enable the controller 103to do so, the memory 104 contains information on the fieldcharacteristics of each coil 11, 12 (curves 21, 22 in FIG. 1) as well asinformation on field distortions caused by the object 3 in the objectspace 2. The controller 103 also has a user input 105, allowing a userto input a selection of an object part 4 of the object 3. For instance,if the object 3 is a human body, the user can for instance select theliver or the stomach or any other organ as object part of interest.Based on this input information, and on the information in the memory104, the controller 103 sets the gains G1, G2 and the phase shifts δ1,δ2 such that the overall B1 field in the selected object part ofinterest is substantially homogeneous.

It is noted that the present invention does not necessarily aim toimprove the homogeneity in the entire object space 2. Instead, thepresent invention aims to improve the homogeneity of the resultantoverall B1 field in a volume of interest. To this end, the presentinvention provides an MRI system 1 which comprises a plurality oftransmit coils 11, 12. Each coil receives a coil drive signal SD1, SD2from a coil drive branch 110, 120. According to an important aspect ofthe present invention, each coil drive branch 110, 120 receives the sameinput signal from a signal generator 101, so that all coils 11, 12receive electrical signal pulses having the same shape, be it that theelectrical signal pulses from different coils may have a differentamplitude and a different phase, controlled by a controller 103 on thebasis of characteristic information in a memory 104 as well as userinput information. The controller is designed to set the respectiveamplitudes and phases in such a way that the resultant overall B1 fieldis as homogeneous as possible in the volume of interest.

It is further noted that the “degree of success” of the control actionby the controller 103 depends on circumstances. Generally speaking, thesmaller the size of the volume of interest 5, the better the homogeneityof the B1 field will be. At any rate, the present invention succeeds inproviding a homogeneity better than if all coils were driven with thesame amplitude and phase.

It should be clear to a person skilled in the art that the presentinvention is not limited to the exemplary embodiments discussed above,but that various variations and modifications are possible within theprotective scope of the invention as defined in the appended claims. Forinstance, although the volume of interest in the FIGS. 1 and 2 is shownas a 2D surface, the present invention is not restricted to 2D volumes;instead, the volume of interest may be a 1D volume or a 3D volume.

1. A magnetic resonance imaging (MRI) system (1) comprising: an objectspace (2) for receiving an object (3) to be examined; a main magnetsystem for generating a main magnetic field in the object space; agradient magnet system for generating gradients of the main magneticfield in the object space; a plurality of transmit coils (11, 12)located adjacent the object space (2); a coil drive circuit (100) forgenerating a plurality of individual coil drive signals (S_(D1),S_(D2)), characterized in that the individual coil drive signals(S_(D1), S_(D2)) are generated by the coil drive circuit (100) so as tohave a substantially identical shape, the system (1) having controllablemeans (110, 120) for individually setting the amplitude and/or phase ofeach of said coil drive signals S_(D1), S_(D2)), and a controller (103)for controlling said controllable means (110, 120).
 2. An MRI system (1)as claimed in claim 1, characterized in that said controller (103) has auser input (105) for receiving a user input signal defining or selectinga volume of interest (5) within said object space (2).
 3. An MRI system(1) as claimed in claim 1, characterized in that said coil drive circuit(100) comprises a signal generator (101) for generating a basic signal(SB) and a plurality of coil drive branches (110, 120) for driving arespective one of the plurality of coils (11, 12), said drive branchesbeing coupled to receive input signals derived from or identical to saidbasic signal (S_(B)).
 4. An MRI system as claimed in claim 3,characterized in that each coil drive branch (110, 120) has its inputcoupled to one output of said signal generator (101).
 5. An MRI system(1) as claimed in claim 3, characterized in that said coil drive circuit(100) also comprises a basic amplifier (102) having an input connectedto an output of said signal generator (101), each coil drive branch(110, 120) having its input coupled to one output of said basicamplifier (102).
 6. An MRI system (1) as claimed in claim 3,characterized in that each coil drive branch (110, 120) comprises acontrollable amplifier (111, 121).
 7. An MRI system (1) as claimed inclaim 3, characterized in that each coil drive branch (110, 120)comprises a controllable phase shifter (112, 122).
 8. An MRI system (1)as claimed in claim 6 or 7, characterized in that said controller (103)is coupled to control said controllable amplifier (111, 112) and/or saidcontrollable phase shifter (112, 122).
 9. An MRI system (1) as claimedin claim 8, characterized in that the system also comprises a memory(104), associated with said controller (103), for storing information onthe field characteristics of each coil (11, 12) and for storinginformation on field distortions caused by an object (3) in the objectspace (2).
 10. An MRI system as claimed in claim 9, characterized inthat said controller (103) is designed to: receive input information atan input (105), said input information relating to a type of object (3)in the object space (2) and a selection of an object part (4); read fromsaid memory (104) individual field characteristics (21, 22) of theindividual transmit coils (11, 12) as well as field distortioncharacteristics of the object (3) in the object space (2); control thesettings of said controllable amplifier (111, 121) and/or the settingsof said controllable phase shifter (112, 122), taking into account saidinformation received at said input (105) as well as said informationread from said memory (104), in such a way that, an overall magneticfield of improved homogeneity, is obtained in a predetermined volume ofinterest (5) corresponding to said object part (4).
 11. An MRI system(1) as claimed in claim 10, characterized in that said controller (103)is designed to control the settings of said controllable amplifier (111,121) and/or the settings of said controllable phase shifter (112, 122)in such a way that an overall magnetic field has a locally substantiallyconstant magnitude at a location within said volume of interest (5),preferably at the center of said volume of interest (5).